Recent studies have focused on the development of ultracapacitors (or supercapacitors) as advanced electrical energy storage devices to increase the efficiency of energy utilization. In most commercial ultracapacitor applications, high surface area carbon has become the leading candidate material in the development of electrochemical ultracapacitors. These devices are also referred to as electrochemical double layer capacitors (EDLC) since the basic mechanism of electrical energy storage is through charge separation in the electrochemical double layer formed at the electrode/electrolyte interfacial regions. When the electrode is biased, a double layer structure is developed with the opposite charge accumulated near the electrode surface. The double layer thickness (d) is related to the Debye screening length in the modified Gouy-Chapman model. The double layer capacitance (c) is related to the surface area, the effective dielectric constant (∈) and the double layer thickness by an inverse linear relationship (C=∈A/d). A typical smooth surface will have a double layer capacitance of about 10-20 μF/cm2. In order to enhance mass storage density, high surface area electrodes are necessary. Thus, for a conducting material with a specific surface area of 1000 m2/g, the capacitance can be increased to 100 F/g.
In most commercial applications, high surface area carbon-based materials have been the material of choice mainly due to their high electronic conductivity and availability at modest cost. A wide range of high surface area carbon-based materials have been investigated, including activated carbon, multi- and single walled carbon nanotubes. The capacitance typically ranges from 40 to 140 F/g for activated carbon, and 15 to 135 F/g for carbon nanotubes. Currently, the best available commercial products reach about 130 F/g.
Those active in the art have pursued several approaches toward improving the charge storage density in carbon-based supercapacitors. These approaches have typically focused on achieving a higher capacitance either by careful thermal, chemical, or electrochemical treatment of the carbon-based material to increase the accessible surface area and surface functional groups, or by extending the operating voltage range beyond the limit of an aqueous electrolyte solution.
Pursuing the first approach, significant effort has been made to maximize the surface area of carbon-based materials. Pursuing the second approach, significant effort has been made to increase the capacitance by modifying the interface. For example, surface functionalization proves to be effective in increasing the pseudocapacitance arising from oxidation/reduction of surface quindoidal functional groups generated during sample treatment. Another widely investigated method enhances the capacitance by coating the carbon-based material with redox active metal oxides such as manganese oxides or conducting polymers such as polyaniline and polypyrrole. With this method, polypyrrole coated carbon nanotubes have been shown to attain a capacitance of 170 F/g, and MnO2 coated carbon nanotubes have been shown to attain a capacitance of 140 F/g, but these composite materials still do not offset the fundamental limitations of the polymer and MnO2, including limited stability and operating voltage range.
Because optimization through surface area and extending the operating voltage range beyond the limit of an aqueous electrolyte solution cannot result in further major improvements, fundamentally new mechanisms need to be discovered to achieve the next significant jump in the storage density of ultracapacitors. The present invention provides one such new mechanism.
Recently, graphene, highly dispersed atom-layer of hexagonal arrayed carbon atoms, has attracted the interest of those seeking to fabricate new composite materials for molecular electronics due to its high conductivity and good mechanical properties. The combination of high electrical conductivity, good mechanical properties, high surface area, and low manufacturing cost make graphene an ideal candidate material for electrochemical applications. Assuming an active surface area of 2600 m2/g and typical capacitance of 10 μF/m2 for carbon materials, graphene has the potential to reach 260 F/g in theoretical specific capacity. However, this high capacity has not been reached because it has proven difficult to completely disperse the graphene sheets and the access all the surface area.
Graphene is generally described as a one-atom-thick planar sheet of sp2-bonded carbon atoms that are densely packed in a honeycomb crystal lattice. The carbon-carbon bond length in graphene is approximately 0.142 nm. Graphene is the basic structural element of some carbon allotropes including graphite, carbon nanotubes and fullerenes. Graphene exhibits unique properties, such as very high strength and very high conductivity.
Graphene has been produced by a variety of techniques. For example, graphene is produced by the chemical reduction of graphene oxide, as shown in Gomez-Navarro, C.; Weitz, R. T.; Bittner, A. M.; Scolari, M.; Mews, A.; Burghard, M.; Kern, K. Electronic Transport Properties of Individual Chemically Reduced Graphene Oxide Sheets. and Nano Lett. 2007, 7, 3499-3503. Si, Y.; Samulski, E. T. Synthesis of Water Soluble Graphene. Nano Lett. 2008, 8, 1679-1682.
While the resultant product shown in the forgoing methods is generally described as graphene, it is clear from the specific capacity of these materials that complete reduction is not achieved, because the resultant materials do not approach the theoretical specific capacity of neat graphene. Accordingly, at least a portion of the graphene is not reduced, and the resultant material contains at least some graphene oxide. As used herein, the term “graphene” should be understood to encompass materials such as these, that contain both graphene and small amounts of graphene oxide.
For example, functionalized graphene sheets (FGSs) prepared through the thermal expansion of graphite oxide as shown in McAllister, M. J.; LiO, J. L.; Adamson, D. H.; Schniepp, H. C.; Abdala, A. A.; Liu, J.; Herrera-Alonso, M.; Milius, D. L.; CarO, R.; Prud'homme, R. K.; Aksay, I. A. Single Sheet Functionalized Graphene by Oxidation and Thermal Expansion of Graphite. Chem. Mater. 2007, 19, 4396-4404 and Schniepp, H. C.; Li, J. L.; McAllister, M. J.; Sai, H.; Herrera-Alonso, M.; Adamson, D. H.; Prud'homme, R. K.; Car, R.; Saville, D. A.; Aksay, I. A. Functionalized Single Graphene Sheets Derived from Splitting Graphite Oxide. J. Phys. Chem. B 2006, 110, 8535-8539 have been shown to have tunable C/O ratios ranging from 10 to 500. The term “graphene” as used herein should be understood to include both pure graphene and graphene with small amounts of graphene oxide, as is the case with these materials.
Further, while graphene is generally described as a one-atom-thick planar sheet densely packed in a honeycomb crystal lattice, these one-atom-thick planar sheets are typically produced as part of an amalgamation of materials, often including materials with defects in the crystal lattice. For example, pentagonal and heptagonal cells constitute defects. If an isolated pentagonal cell is present, then the plane warps into a cone shape. Likewise, an isolated heptagon causes the sheet to become saddle-shaped. When producing graphene by known methods, these and other defects are typically present.
The IUPAC compendium of technology states: “previously, descriptions such as graphite layers, carbon layers, or carbon sheets have been used for the term graphene . . . it is not correct to use for a single layer a term which includes the term graphite, which would imply a three-dimensional structure. The term graphene should be used only when the reactions, structural relations or other properties of individual layers are discussed”. Accordingly, while it should be understood that while the terms “graphene” and “graphene layer” as used in the present invention refers only to materials that contain at least some individual layers of single layer sheets, the terms “graphene” and “graphene layer” as used herein should therefore be understood to also include materials where these single layer sheets are present as a part of materials that may additionally include graphite layers, carbon layers, and carbon sheets.
Traditionally conductive graphene sheets have produced by mechanical exfoliation. By nature the graphite surface is hydrophobic. Oxidation of graphite followed by exfoliation has been shown to produce more soluble graphene oxide, but with a lower conductivity. Reduction of graphene oxides to increase the conductivity significantly reduces the solubility (<0.5 mg/mL) and makes the material vulnerable to irreversible aggregation.
Following the research from carbon nanotubes, two main methods to improve surface properties of graphene have been investigated. The first approach is through surface functionalization of reduced graphene oxides in order to make soluble and stable graphene possible for materials process. For example, functional groups (e.g., —CH3, —SO3 group) are covalently attached to graphene surfaces through oxygen functionality (—O —, —COOH), but this process also incorporates defects on sp2 conjugation of carbon atoms, which affect the intrinsic unique properties such as high conductivity.
The second approach is non-covalent functionalization using surfactant, polymer or aromatic molecules. In general a good electrode material needs to meet some key requirements: good wetting for the electrolyte or catalyst, a good conductive pathway throughout the electrode materials, and a continuous porous network for rapid diffusion and mass transport. To date, efforts to produce materials using the second approach have not approached the theoretical properties of graphene based materials. The present invention overcomes those shortcomings.